Fuel-fired heating appliances, such as, for example, gas-fired residential and commercial water heaters, are often formed to include a vertical cylindrical water storage tank with a gas burner disposed in a combustion chamber below the tank. The burner is supplied with fuel gas through a valved gas supply line, and combustion air through an air inlet flow path providing communication between the exterior of the water heater and the interior of the combustion chamber.
Water heaters of this general type are extremely safe and quite reliable in operation. When gasoline or other flammable liquids are stored or used improperly in proximity to the water heater, however, it is possible that flammable vapors may become entrained in the air intake of the water heater. Such vapors might cause secondary combustion to occur within the confines of the water heater combustion chamber. Dangers are equally present with other types of devices, such as hot surface igniters and/or pilot flames. Any source of spark, flame or high temperature (e.g., auto-ignition temperature devices) can cause fires and/or explosions. If vapors are present, shutting down the ignition system is the most critical response, while shutting down the fuel flow is also desirable.
In residences where appliances such as gas water heaters are located in close proximity to fuel sources, such as gasoline containers, there is a potential for the flammable vapors to be ignited by devices such as a water heater pilot flame. A vapor sensor mounted to the water heater would ideally shut down the pilot if gasoline vapors are detected. Present sensors capable of detecting flammable vapors can be rendered ineffective or deliver false alarms when the sensing element is damaged, exposed to air flow, dust, liquids, oils, or other contaminants.
In view of the foregoing, a continuing need exists for the preclusion of fuel flow to the appliance when extraneous flammable vapors are present exteriorly adjacent the appliance. In particular, a continuing need exists for improved sensors that can sense ignitable gasoline vapors near appliances or devices such as water heaters, clothes dryers, and the like in order to shut them down before they cause fires or explosions. Such vapor sensors should be insensitive (or compensated) for other environmental effects (altitude, humidity, temperatures, etc.) that might cause either false alarms or unsafe conditions.